Firtly, the goldfish will quickly outgrow that sized tank and in the meantime you will stunt their growth. I would like to suggest you keep the fantails in it and put the comets in a pond - a much more suitable environment for them. Comets need space to swim as they are quick and lively. The fantails are much slower and don't need so much room to swim.
Goldfish nibble at all plants and some are worse than others for doing it. I collect oxygen weed from the local river as it is free and easily replenished. You might also try Java fern and water sprite although they will tug on the sprite endlessly then eat the exposed root - which kills off the rest of the plant :roll:
Just drop the lettuce in - wash it first to get rid of any pesticides which may be present.